Capacitive pressure sensors are well known and employed in capacitance transducers, microphones, rupture discs, resonators, vibrators and like devices. Many of the applications for such capacitive pressure sensors require that the sensors be extremely small, for example, of the order of about eight millimeters by eight millimeters (8 mm.times.8 mm) or less.
Silicon capacitive pressure transducers are known in the art. For example, U.S. Pat. No. 3,634,727 to Polye discloses one type in which a pair of centrally apertured, conductive silicon plates are joined together with a eutectic metal bond, such that the silicon disc plates flex with applied pressure, changing the capacitance of the aperture interstice and providing a capacitive-type signal manifestation of pressure magnitude. This form of pressure transducer thus relies on the pressure-induced deflection of a thin diaphragm, in which the diaphragm deflection as a function of fluid pressure causes a variation in the distance between a pair of surfaces which effectively form the plates of a variable capacitor. Other examples of such silicon pressure sensors or transducers are included in the U.S. patents listed below.
In many high accuracy applications typical of those encountered in aerospace products, long-term drift (for example 20 years and longer) at an elevated temperature (for example 120.degree. C. and higher) of the pressure sensing element limits the overall achievable system accuracy.
In an exemplary prior art, silicon-glass-silicon pressure sensor design of the sandwich type (note FIGS. 1 and 2), used as an exemplary baseline in the disclosure of the present invention, the dielectric spacer between the diaphragm and base, particularly in the upwardly extending wall support area formed by the dielectric layer at the operative periphery of the sensor, comprises approximately fifty (50%) percent of the total capacitance of the sensing element. In the present invention aging or drift in the electrical properties of this dielectric wall spacer, typically made of borosilicate glass, located typically at the periphery of the device, generally identified as "C.sub.p ", has been identified in the invention as being the major contributing factor to the drift of the sensing element.
As can be seen in FIGS. 1A and 1, the exemplary prior art silicon-on-silicon pressure sensor or transducer 10, which typically is generally square in its exterior configuration but often at least generally and preferably circular or cylindrical in shape for its inner, operative substructure, generally identified as "C.sub.c " in FIG. 1, includes an upper, conductive, square, flexible, appropriately doped, silicon diaphragm 11 and a lower or bottom, conductive, appropriately doped, silicon base or substrate 12 with a non-conductive dielectric layer and spacer 13 (made of, for example, borosilicate glass) between them, a closed, evacuated, hermetically sealed, reference cavity, chamber or interstice 14 being formed between the two silicon layers 11, 12. The chamber 14 is typically at a zero vacuum or can be sealed at a higher reference pressure, at which reference level the diaphragm 11 is parallel to the silicon substrate 12, with typically a two micrometer spacing between the two.
It should be understood that the simplified drawings hereof for practical purposes of illustration are not at all to relative scale, as the glass wall or spacer 13/16 is only typically nine micrometers high, in contrast to the thicknesses of the silicon layers 11 and 12, which typically are eight thousandths (0.008") of an inch and fifty thousandths (0.050") inches thick, respectively, for an exemplary fifty (50 psi) pounds per square inch pressure measuring unit.
A centrally located, typically circular pedestal or mesa 12A extends into the typically generally cylindrical, closed chamber 14 with a thin, insulating layer of glass 13A (not shown in FIG. 1A) covering the top of the mesa. Due to the thinness of the layer 13A, typically only a half of a micrometer, which is usually deposited after the relatively high wall 16 (typically nine micrometers), it does not substantially contribute to any long term drift problems of the sensor 10, and its changing characteristics over the long term (e.g. 20 years) can be ignored, in so far as the present invention is concerned.
As the external ambient pressure on the outside of the sensor 10 varies, the diaphragm 11 flexes, causing the spacing between the silicon layers 11 and 12, serving as capacitive plates, to change, in turn changing the capacitance of the sensor. This change in capacitance as a result of a change in the exterior pressure on the exterior surface or upper-side 17 of the diaphragm 11 is used as a measure of the pressure and its changes.
Conductors or electrodes 18A and 18B (not illustrated in FIG. 1 for simplicity purposes) to the silicon layers 11 and 12 are included for connecting the transducer or sensor 10 into an appropriate circuit, many of which are known to the art, which measures its changing capacitance as a function of the pressure. The varying pressure on the exterior, sensing surface 17 of the elastic silicon diaphragm 11, causing the diaphragm to flex, changes the value of the interstitial capacitance between the diaphragm and the electrode to the lower silicon substrate 12, which transduces the applied pressure to a measurable electronic signal. Typically, as noted above, there is about an exemplary two micrometer gap between the inner, lower, underside surface of the diaphragm 11 and the top or upper-side of the mesa 12A, when the sensor is at its zero or reference pressure, to allow room for the diaphragm to flex inwardly toward the mesa 12A, as the pressure increases.
Critical stress region(s) 15 occur(s) at the inner, edge interface between the flexible silicon diaphragm 11 and the wall(s) formed by the vertically extended, peripheral portions 16 of the dielectric spacer 13, due to the flexing movement of the diaphragm about the region(s), as the ambient or sensed pressure changes. The wall(s) 16 might typically have a horizontal, lateral or radial thickness of, for example, thirty-six thousandths (0.036") of an inch with a height of, for example, nine (9) micrometers, while the separately applied, insulating, mesa layer of glass is only about a half a micrometer thick. The mesa 12A extends up from the main surface of the silicon substrate 12 an exemplary six and a half micrometers, while having an exemplary diameter of one hundred and fifty thousandths (0.150") of an inch.
The silicon diaphragm 11 and the silicon base 12 may typically be square [with corners removed for the purpose of providing access for electrical contacts to the layer(s), as illustrated], having a horizontal length of an exemplary two hundred and sixty thousandths (0.260") of an inch on an edge, while the spacer wall 16 can have an inner diameter of an exemplary one hundred and ninety thousandths (0.190") of an inch. The outer, side surface of the wall spacer 16 can either follow the basic square configuration of the silicon layers or having an outer circular configuration.
As can be seen in FIG. 1A, a transition piece 18 is bonded through an exemplary glass layer 20 to the upper, exterior surface 17 of the diaphragm 11 and includes a pressure port 19, through which the pressure to be sensed is communicated to the diaphragm. In turn the sensor 10 is appropriately mounted for use in the desired application. These packaging aspects form no part of the present invention.
An exemplary, prior art, three plate, silicon-glass-silicon (SGS) device is particularly described in assignee's U.S. Pat. No. 4,467,394 of Grantham & Swindal. Due to the relative sizes and electrical characteristics of the three plates, the dielectric wall spacer 16 at the peripheral, outer, peripheral regions of the device can account for approximately fifty (50%) percent of the total capacitance of such a prior art sensor, that is, about one (1) part in two (2). This peripheral capacitance is considered parasitic and undesired, as it is pressure insensitive.
With the structural designs of the present invention, the contribution of the capacitance "C.sub.p " of the peripheral, supporting, dielectric spacer wall to the overall capacitance of the sensor is reduced, for the baseline example, from about one (1) part in two (2) of the prior art down to, for example, a maximum of about one (1) part in six (6) with a minimum of about one (1) part in ten (10) invention; that is, from about fifty (50%) percent in the prior art down to, for example, about sixteen (16%) percent to about ten (10%) percent, or lower, in the invention. Accordingly, the overall sensor element drift rate is reduced by a commensurate amount.
Other prior art approaches may have achieved more favorable ratios than one (1) part in two (2) by the introduction of complex lead-throughs or by the substitution of an insulating structure for the silicon base of the existing design. However, these approaches are either more costly, because of the complexity they introduce, or they compromise the sensing element performance because of the gross introduction of dissimilar materials having expansion coefficients which do not ideally match.
A further approach is that of U.S. Pat. No. 4,597,027 of Lehto (issued 06/24/86), which includes recessing the dielectric layer down into the silicon substrate, so that it does not extend above the upper plane of the silicon substrate and no longer serves as a wall spacer, generates a number of other problems, including diminished precision, which makes its approach somewhat undesirable. This approach also requires that the peripheral edges of the diaphragm be extended down to, in essence, provide the wall spacing function of the glass wall spacer 16 of FIGS. 1-4 hereof, which approach, inter alia, causes problems with respect to the flexing of the diaphragm. In contrast the diaphragm 11 of the SGS "sandwich" sensor combination of the invention can be, and preferably is, flat, that is, it is uniform in thickness across its lateral extent, except for the possibility of providing an indentation of small width for a diaphragm hinge, as discussed in the co-pending application entitled "Capacitive Pressure Sensor With Hinged Silicon Diaphragm" (R-3288hs-ed) referred to above.
Some exemplary prior art patents in the field of capacitive pressure sensors or transducers, owned by the assignee hereof, are listed below:
______________________________________ Patent Issue No. Title Inventors Date ______________________________________ 4,530,029 Capacitive Pressure C. D. Beristain 07/16/85 Sensor With Low Parasitic Capacitance 4,517,622 Capacitive Pressure B. Male 05/14/85 Transducer Signal Conditioning Circuit 4,513,348 Low Parasitic D. H. Grantham 04/23/85 Capacitance Pressure Transducer and Etch Stop Method 4,467,394 Three Plate Silicon- D. H. Grantham 08/21/84 Glass-Silicon J. L. Swindal Capacitive Pressure Transducer 4,463,336 Ultra-Thin Microelec- J. F. Black 07/31/84 tronic Pressure T. W. Grudkowski Sensors A. J. DeMaria 4,415,948 Electrostatic Bonded, D. H. Grantham 11/15/83 Silicon Capacitive J. L. Swindal Pressure Transducer 4,405,970 Silicon-Glass-Silicon J. L. Swindal 09/20/83 Capacitive Pressure D. H. Grantham Transducer ______________________________________